1. Technical Field
This invention relates to circular saw and bandsaw machines, and incorporates a method of controlling the rate at which work is fed into the saw blade based upon performance of the saw blade within predetermined levels of stability.
2. Background
Circular saw and bandsaw machines have long been used as economical means for cutting wood, metal and other materials. In recognition of the high costs for raw material and labor, automatic/computer control of work feed rates and sawing accuracy becomes of paramount importance. Optimized automatic control of work feed rates and saw blade stability keeps material and production costs down. The use of thinner saw blades and smaller rough sawn dimension sizes can conserve natural resources in the wood products industry, and reduce material waste in all industries which use circular saw and bandsaw machines in the manufacturing process. In the lumber industry, current production methods result in a larger than necessary amount of waste in order to manufacture finished dimension lumber. Reduction of this waste requires the solution of several technical problems.
The first problem is the rate at which the work is fed into the saw blade. The work is either manually or automatically fed into these sawing machines. In manually fed machines, the operator listens to the sound of the saw blade and varies the feed rate by judgment, frequently not realizing that the saw blade is being overfed. Sawing machines having automatic feed systems, use predetermined feed rates as a function of thickness of work (depth of cut). These predetermined feed rates do not consider any variable except depth of cut. This automatic feed system permits overfeeding and in some cases underfeeding of the saw machine, resulting in erratic lateral movement of the saw blade and loss of control of the saw line. Slower than optimum feed rates are required to compensate for the variable densities of wood encountered from summer to winter, density changes within the same log, partially frozen logs, and the sharpness of the saw blade cutting teeth. The variables all require on-line adjustments to work feed rates.
The second problem is the target size of the rough sawn work, which must be maintained large enough so that finished lumber is not undersized. This excess material, which is later removed to produce finish dimension lumber, represents waste. Uncontrolled lateral deviations in the saw lines during the cutting operation require larger rough sawn target sizes. These saw blade movements have several causes: misalignment of saw blade guides, normal saw blade tooth wear, bending or uneven dulling of saw teeth, and knots in the saw log. These typical conditions can cause lateral instability of the saw blade, with resultant deviations of the saw line.
The third problem is offsetting of the saw blade from the desired saw line. If the saw teeth are dulled by sand, gravel or other foreign objects embedded within the material being cut, offsetting the saw line from minor deviation of 0.005 to a major of 0.080 of an inch. When a saw blade runs in an offset condition and encounters a knot, or is substantially over-fed, it is possible for the saw blade to run completely out of the work. This ruins the saw blade tension, requiring hours of bench work to bring the saw back into proper tension so that it will again cut straight and accurately. The saw blade could also disintegrate, destroying itself and surrounding equipment, thus requiring down time to repair the damage. The safety of personnel is also placed in jeopardy if the saw blade disintegrates.
The fourth problem is the width of the saw cut, or kerf. Reduction of the saw blade gauge/thickness, and of side clearance, (the distance the tooth extends beyond the side of the saw blade body), decreases the width of the kerf. Heavier gauges and larger side clearance are currently used to protect the saw from the instability effects of excessive feed rates.
The fifth problem encompasses other considerations that directly affect optimized cutting efficiency, such as saw blade design, saw blade strain, and guide pressure.
At the present time, these considerations are being addressed by using larger rough sawn lumber target sizes, thicker saw blades, and larger kerf dimensions. Numerous attempts through the years have been made to solve these problems, with varied success.
1. Saw blade strain devices such as weight and lever or high strain pneumatic systems have improved saw blade performance. Some improvement in saw blade stability has been obtained, and higher feed rates achieved.
2. The use of pressure guides provides an additional increment of saw blade stability. These devices are commonly used in the wood products industry.
3. A control system utilizing the saw blade sagging angle in the direction of the work feed was the basis for U.S. Pat. No. 4,437,367, which was issued to Karl Hauser. This system works well in small bandsaw machines, but will not function adequately with the larger bandmill machines which have wider saw blades. This patent applies to bandsaw machines that hinge and move to the work in lieu of the work feed system common in larger sawing machines in which the work is fed into the machine.
4. Utilizing the pressure imposed by the work on the back of the saw blade to control work feed rates, as in U.S. Pat. No. 3,680,417 issued to John R. Wells, has merit when using small bandmill machines which use throw-away saw blades. Large band saw machines have blade widths exceeding two inches, and the same problems exist with this patent as with the saw blade sagging angle control system.
5. Utilizing a control system as shown in my U.S. Pat. No. 4,644,832, which uses a mean or averaged signal proportional to the lateral position of the saw blade for slowing down the depth of cut entry speed. This patent allows for the work entry feed speeds to be set higher than normal and the control logic to use xe2x80x9cslow down stepsxe2x80x9d to reduce work feed speed in the event of unacceptable lateral movement of the saw blade. However, using this system, once a slow-down step has been made, the speed remains slowed down for the entire length of a log or cant being sawn, which lowers production output. This prior art does not address variable conditions within the same work piece, such as a log wherein the sawing conditions may vary significantly from one end to another, for example, from the butt end to the top end, as they relate to material density, sap rings, moisture content and even temperature.
In addition, my prior U.S. Pat. No. 4,644,832, does not take into consideration changes in zero reference signals which can change, in a 2-hour period, as much as 0.020 inches as a result of wear during cutting.
Finally, this prior art patent utilizes only processed signals, and in the event of significant lateral deviation of the saw blade, the prior art system does not react quick enough to prevent damage to the saw blade and/or the work piece because of the processing time required to condition the signals.
Accordingly, it is an object of this invention to provide a saw control system which is responsive to changing cutting condition so as to optimize saw cutting conditions.
These objects are achieved in a control system which continuously monitors the lateral displacement of the sawing blade from its designed straight line position by means of a sensor which is installed at a fixed known position relative to the blade. The signal is processed into an averaged signal proportional relative to a zero reference point, and thus proportional to the lateral motion of the blade for any given period of time. This signal is then compared to a plurality of reference signals to monitor the lateral position of the blade, and for purposes of adjusting the feed rate at which the material being cut is presented to the saw. The reference signals are provided in pairs, with the first being those proportional to a predetermined acceptable lateral motion range, followed by a first, second and third range reference signals. Since every sawing application is different, the set points for the reference levels are empirically determined for the particular application.
The work feed motor has a variable feed rate capability. An empirically determined table assigns to each depth of cut a thickness designation and an initial entry feed rate assignment. These entry feed rate assignments are either determined empirically or derived from existing tables published for most particular saw blade configurations. The greater the depth of cut, the slower the initial entry feed rate will be. Also provided are a plurality of increasing and decreasing feed rate steps for each initial entry feed rate. These steps can be fixed or percentage based so that the greater the depth of cut, the less the feed rate step increases, and the smaller the depth of cut, the greater the increases and decreases.
Prior to the material being cut being presented to the saw blade, it is first passed through a depth of cut thickness measurement device, where the thickness is measured and compared to the entry feed rate table and the appropriate initial entry feed rate is selected.
As the material engages the saw blade, the lateral position of the saw blade is continuously monitored by a sensor and compared to the acceptable lateral motion reference signals. If the blade motion signal remains within the acceptable lateral motion signals for a predetermined period of time, then the output to the work feed motor will be increased one step. This sequence of events will continue as long as the lateral position of the saw blade remains within the acceptable lateral motion range.
If the lateral position of the saw blade moves outside of the acceptable lateral range, but not above or below the first reference point, then the increase in feed rate steps will stop. If the lateral position of the saw blade moves beyond the first reference range, the controller will signal the work feed motor to drop its operating speed one step. If the lateral position of the saw blade remains outside of the first reference range for a specified period of time after dropping a speed step, the controller will signal the work feed motor to continue dropping steps until the lateral saw blade motion drops within the acceptable lateral motion range.
If the raw wave signal from the sensor indicates that the lateral motion of the second reference point range, the feed rate signal from the controller will automatically drop a predetermined number of steps, depending upon a pre-selected set point parameter.
If the raw wave signal from the sensor indicates that the saw blade has moved laterally past a third reference point, then the controller will signal the work feed motor to immediately drop in feed rate to a slow speed to the depth of cut being sawn.
In a like manner, the controller can be used to step up or down the blade drive motor and thus provide adjustments for tip speed for the blade.
The position of the saw blade is also monitored, using the sensor. During those periods of time when the saw blade is not cutting, these readings are used to periodically reset the initial zero reference point so as to compensate for changes in position resulting from wear on the saw and guide blocks.